1. Field of the Invention
The present invention concerns a device for the compressing of a gaseous medium and systems that apply such compressing devices, such as energy generating systems, gas separators, compressors and compressor systems for natural gas, air, and chemicalsxe2x80x94such as ammoniac.
2. Brief Description of the Prior Art
Compressing a gaseous medium requires compression work, which work is directly proportional to the absolute temperature of the medium to be compressed. This means that compression work may be reduced by cooling the medium before and during the compression, and in the event of recycling, also after compression. This applies also if the medium is compressed successively in various stages. The main objective is a virtually ideal or quasi-isothermal compression.
The medium is cooled by introducing a liquid evaporation agent (usually water). The evaporation agent is introduced in the form of droplets that evaporate. The heat required for evaporation is provided by the medium, which consequently cools.
In principle, it is not necessary that the sprayed droplets of the evaporation agent evaporate completely. Incomplete evaporation of the droplets, however, may result in droplets of the evaporation agent coming into contact with the interior of the compressor unit, which may lead to erosion and corrosion of parts of the compressor unit.
The objective, therefore, is to introduce as tiny droplets as possible (1-10 xcexcm). The smaller the droplets, the likelier the possibility that they will evaporate completely, but also the less likely that they will come into contact with the structure of the compressor unit. However, if the medium has a high velocity and/or the air residence time in the compressor unit is short, there usually is insufficient time for full evaporation.
DE-A-41 14 678 relates to a method for the atomisation of a fluid for a gas turbine. The atomisation of the fluid has to take place over the entire length of the compressor at a spraying pressure of 5-20 bar above the compressor pressure. The maximum quantity of fluid to be atomisedxe2x80x94water in particularxe2x80x94lies between 0 to 0.2 kilogram per kilogram air and may not be exceeded. Finally, it is indicated that the manner wherein the fluid is atomised in the compressor has not yet been constructively solved.
U.S. Pat. No. 4,478,553 relates to the isothermal compression in the compressor of a gas turbine. Atomising means are strategically positioned in the rotor structure. The water to be dispersed is not pre-heated and the size of the droplets of the atomised evaporation agent is preferably maintained at between 2 and 10 xcexcm.
U.S. Pat. No. 5,388,397 relates to a method for operating a turbocompressor, whereby air is compressed in two stages and cooled in between in an intercooler. The warm water of the intercooler is cooled in subsequent evaporating vessels and the resulting steam is transferred very compactly to the environment by spontaneous evaporation according to a flash method. The evaporated amount is made up to by a corresponding amount of fresh water.
EP-A 0 821 137 describes a system for generating energy, whereby the gas to be compressed is cooled by atomising water droplets with a drop size of 1-5 xcexcm. Under certain conditions, however, the required flow rate of atomised water droplets is too small.
The present invention aims to provide a compressor device in which a gaseous medium may be compressed at relatively low temperatures by applying very small droplets of evaporation agent (the median is smaller than 5 xcexcm, generally smaller than 3 xcexcm, preferably smaller than 2 xcexcm, e.g. 1.2 xcexcm) whereas sufficient flow rate of this type of atomised droplets may be generated in dependency of the flow rate of the medium to be compressed.
Simultaneously, the present invention aims to provide a very adequate manner of cooling a gaseous medium, so that the cooling capacity of existing or required gas coolers (intercoolers) may be reduced or that they may be replaced.
This is achieved according to the invention by a device for compressing a gaseous medium, generally including a compressor unit provided with a medium inlet, an outlet for the compressed medium and of means for atomising a liquid evaporation agent in the medium, wherein the atomising means have at least one flash atomisation unit, mounted and arranged such, that the atomised evaporation agent fragmentises by the formation of gas in the atomised evaporation agent.
The atomising means of this compressor unit includes an inlet for evaporation agent and an outlet for evaporation agent into the gaseous medium line. It is possible that this gaseous medium still has to be compressed, is in the process of being compressed or has already been compressed. In the latter instance, the compressed medium may still be added to a subsequent compressing unit or may in part be recirculated. The atomising means usually also contain a very large number of atomisers via which the evaporation agent is sprayed into the gaseous medium.
In principle, any known type of atomiser may be used in the flash atomisation unit. Suitable ones are, for example, swirl atomisers, slot atomisers, orifice atomisers, rotating bowl atomisers and, if necessary, pen atomisers. Of importance is only that the atomiser gives off droplets or a film of evaporation agent to the gaseous medium, under circumstances changed to such extent that flash atomisation takes place subsequently. Flash atomisation means that the liquid evaporation agent arrives in the gaseous medium under such conditions that as a result of the pressure drop over the atomiser, boiling bubbles or gas bubbles are generated in the droplets or film of the evaporation agent, i.e. gas or vapour is formed in the evaporation agent. This so-called flashing or precipitation results in the explosion or fragmentation of the droplets or film of the evaporation agent as a result of the sudden partial boiling or gas precipitation. Such fragmentation results in the generation of very tiny droplets of evaporation agent in the gaseous medium. After fragmentation, the median size of the evaporation agent is less than 5 xcexcm, in general smaller than 3 xcexcm, preferably smaller than 2 xcexcm, e.g. 1.2 xcexcm.
This means that atomisers may be employed in the atomising means insofar as they result in droplets of the said median size after fragmentation. Of importance in this respect is that the atomising means, the flash atomisation units in particular, are mounted and arranged such that the atomised evaporation agent fragments by the generation of gas in the atomised evaporation agent.
Preferably, a flash atomisation unit is used that is provided with swirl atomisers. In such a known swirl atomiser, the evaporation agent is put in swirling motion in a swirl chamber. The swirling evaporation agent exits via an outlet. It has appeared that the thickness of the exiting layer of evaporation agent is only a fraction (e.g. 10%) of the diameter of the outlet passage. The subsequent flash fragmentation results in droplets having (dependent on the pressure drop, temperature and diameter of the outlet passage) a median size of 5 xcexcm or less. Because of the reduced size of the droplets of evaporation agent, there will be less risk of contact with the interior of the compressor unit, which means that the entire length of the compressor unit can be cooled.
It will be clear that in order to realise this fragmentation, it is important that the conditions (in particular the changing of conditions) under which the evaporation agent is atomised in the gaseous medium must be optimal for fragmentation. Important conditions for flash fragmentation are the temperature of the evaporation agent, the atomisation pressure under which the evaporation agent is atomised in the gaseous medium, and the outlet passage diameter. Therefore, the flash atomisation unit preferably, has means for adjusting the temperature of the evaporation agent and/or the atomisation pressure.
As indicated above, known atomisers may, in principle, be used in the compressor device according to the invention. These atomisers may release the evaporation agent in the gaseous medium in a direction which is either traverse or parallel to the flow of the gaseous medium. The atomised evaporation agent may possess a radial or axial component vis-a-vis the gaseous medium. A radial component is important in order to avoid coalescence of the fragmentised droplets of evaporation agent and may be realised e.g. by applying a swirl atomiser. An axial component is important in order to transfer the energy of the evaporation agent as much as possible to the gaseous medium, such that the pressure drop is low or even negative. In the event the existing compressors or compressor systems for instance energy installations are retrofit, pre-position is provided for a spraying rack with swirl atomisers. This spraying rack should preferably be positioned close to the medium inlet of the compressor so that there is hardly any opportunity for droplet coalescence or heating of the medium. Under similar conditions it is also possible to include the atomisers in the blades of the compressor and to atomise from the stator or rotating compressor blade. The swirl atomiser and the slot atomiser or orifice atomiser in particular are preferred here because they are of a very simple construction and are quite easy to miniaturise. Accordingly, very large numbers of atomisers may be implemented in advance without requiring too extensive changes in the existing compressor device, thus enabling an optional but also large flow rate of fragmentised evaporation agent. A retrofit set up in this manner effectively reduces both the compression discharge temperature and the compression work.
If, furthermore, the means for adjusting the temperature adjusts the temperature of the evaporation agent preferably to or near the critical temperature, the evaporation agent attains a surface tension which is virtually or precisely 0 N/m2. This means that little or no further energy is required to atomise the liquid, so that the droplet size will be extremely small (a median droplet size of up to 0.1 xcexcm is possible) and the use of other means for reducing the surface tension may be dispensed with. Depending on the amount of liquid evaporation agent that is atomised in the medium and the distance to the medium inlet of the compressor, the temperature of the medium/evaporation agent to be compressed may increase, e.g. from 15xc2x0 C. to 23xc2x0 C. and 30xc2x0 C. at a final water content in the medium that leaves the compressor unit of 10 and 18 mol % respectively. Therefore, it is advantageous if the temperature of the evaporation agent to be atomisedxe2x80x94water in particularxe2x80x94before atomisation is as low as possible. Finally, because of the extremely small size of the droplets, an optimal and maximum evaporation and consequently cooling will occur, as a result of which the compression work is minimal and therefore the forming of NOx as well.
Besides the aforementioned physical conditions for fragmentation it is also possible to promote fragmentation by adding chemical or physical additives to the evaporation agent. It is therefore preferable to add agents to the evaporation agent which reduce its surface tension, thereby reducing the energy required for the fragmentation. Agents that can be used to reduce the surface tension are detergents and the like. Preferred surface tension reducing agents are those that do not only reside at the interface of the evaporation agent and medium but that are also virtually homogeneously distributed throughout the evaporation agent (droplet or film). Thus, a reduced reduction of the surface tension is not required after atomisation and prior to fragmentation as a consequence of diffusion. Under such conditions the use of fatty acids, shortened fatty acids in particular, is preferred or, possibly, alcohol, e.g. methanol or ethanol. Use of the latter substances is especially preferred in the event they are to be added to gaseous mediums that are subsequently used in a combustion process. Thus, the risk of these additives negatively influencing the combustion process is avoided.
According to another preferred embodiment, the evaporation agent, generally includes a number of evaporation substances that each feature different boiling points. In particular, as the result of a pressure drop when the flash atomisation unit is passed, the vaporisable substances with the lowest boiling points will be the first to evaporate in a flash, forming boiling bubbles, as a result of which the remaining (liquid) evaporation agent will explode or fragment into small droplets. The mixture may e.g. be a mixture of water and carbon dioxide or a mixture of water and carbon monoxide.
The addition of a vaporisable substance having a lower boiling point also results in a further reduction of temperature in the atomised droplets. The atomisation of water, which is saturated with carbon dioxide (approx. 7% by wt at 150 bar) at 150 bar and 15xc2x0 C. results, when it is suddenly expanded to 1 bar, in a lowering of the temperature to 12.5xc2x0 C.
In principle, the compression device according to the invention may be applied under all kinds of conditions, in particular under conditions requiring isothermal or quasi-isothermal compression for reasons of efficiency, and such under conditions that leave little time for evaporation as a result of the limited residence time before, in or after the compression unit.
The compression device according to the invention turns out to be well applicable in systems for generating energy, such as compressor units provided with gas turbines as well as installations for gas separation or combustion engines. In principle, the invention is applicable to all gases that have to be compressed, such as natural gas, ammoniac, air, nitrogen and oxygen, hydrogen, synthesis gas, carbon dioxide and inert gases.
The compression device according to the invention may also be used in a rotating or piston engine, such as a combustion engine, e.g. a gas engine, diesel engine and Otto engine. The piston compression in a piston compressor or during the compression stroke in a combustion engine may be reduced in work in the same manner as in the axial or radial (gas turbine) compressor by applying quasi-isothermal compression. In a diesel engine with turbocharger atomisation may take place both before and in the turbocharger and before and in the compression chamber. The finely atomised water will evaporate and the temperature and the compressor-work will be lower than with adiabatic compression.
As indicated above, in a combustion engine the flash atomisation unit is preferably incorporated in the separate compression chamber or compression unit. Thus, quasi-isothermal compression may occur during the compression stroke of the combustion engine. A heat exchanger is arranged between the compression chamber or unit and the combustion chamber of the combustion engine which is in heat-exchanging contact with an exhaust outlet of the combustion engine. Thus it is possible to recuperate heat in the cool, compressed air from the heat of the exhaust gases.
The features stated and other features of the compressor device and of the systems in which such devices are used, will be given below as examples without restricting the invention thereto.